1. Field of the Invention
The present invention relates to a bandwidth compressing/reproducing system which subjects a television signal to bandwidth compression and then subjects the bandwidth-compressed; television signal to bandwidth reproduction.
2. Description of the Related Art
The NTSC system is one of color television broadcast systems. This NTSC system can be considered an excellent system as it has a compatibility with a monochromatic television broadcast system and has a sufficient performance as a color television broadcast system. This is proved by its employment in Japan, the U.S.A. and other countries.
The image quality in the NTSC system has been significantly improved since the one achieved in the beginning due to great efforts having made continuously by both sender and receiver sides in its own long history.
As a recent trend toward larger display devices, however, there is a growing demand for still higher image quality in the NTSC system.
An IDTV (Improved Definition Television) system, for example, serves to improve image quality in the NTSC system. This IDTV system improves image quality by effectively using a transmitted color television signal of NTSC system (hereinafter referred to as NTSC signal). The IDTV system can be realized by the recent progress in digital technique.
The IDTV system can significantly improve image quality as compared with the conventional analog system.
Since the IDTV system is premised on the NTSC system, however, the improvement on image quality is regretfully restricted by the standards of the NTSC system.
The following are examples of the restrictive items:
(1) Horizontal to vertical screen ratio (aspect ratio).
(2) Horizontal resolution.
According to the current NTSC system, the aspect ratio (1) is 4 : 3; therefore, the aspect ratio for the IDTV system is also restricted to 4 : 3.
The horizontal resolution (2) is 330 TV lines due to the horizontal bandwidth of the current NTSC system being restricted to 4.2 MHz. The IDTV system inevitably has a restricted horizontal resolution of 330 TV lines.
With regard to the aspect ratio (1), recent surveys indicate that users prefer a ratio of 5 : 3 or 16 : 9. (See "Broadcast System," edited by Nippon Hoso Kyokai (NHK), page 80.) There is a possibility of employing an aspect ratio of 16 : 9 in the HDT (High Definition Television) system (see CCIR Report 801-2).
The horizontal resolution (2) should be improved in order to maintain the balance with the current vertical resolution. Because the vertical resolution can be improved up to 450 TV lines by the IDTV system.
From the above, it is desirable to improve the aforementioned two restrictive items while keeping the compatibility with the current NTSC system.
An example of such a system is the SLSC system as disclosed in "A Compatible High-Definition Television System (SLSC) with Chrominance and Aspect Ratio Improvements," by Joseph L. LoCicero, SWPTE Journal, May 1985.
According to this SLSC system, there are bands for two channels per station provided: one channel transfers a signal basically similar to the current television signal and the other transfers a added signal for improvement of image quality.
As two channels are specifically used per station according to this system, the efficiency in channel use is not high. Particularly, with channel assignment being close to its limit as in Japan, difficulties apparently lie in utilizing such a system. With regard to in-station or station-to-station transfer, existing television broadcast machines do not have bandwidths of 10 MHz as defined by the SLSC system, thus necessitating investment in new equipments.
In this respect, it is desirable to use a system which can transfer an added signal within a one-channel bandwidth. Further, in view of the compatibility of the existing television broadcast machines such as video tape recorders and transmitters, it is desirable that the broadcast system can multiplex an added signal around a base bandwidth of 4.2 MHz.
Such a system is disclosed in "Extended Definition TV Fully Compatible with Existing Standards," by T. Fukinuki et al., IEEE Tr. on Communication Vol. COM-32 No. 8, August 1984. The disclosed system uses a spectrum region unused for a still picture to multiplex a detail component of luminance (component with a horizontal bandwidth of about 4 to 6 MHz) onto an NTSC signal, in order to improve the resolution of a still picture. As the unused spectrum region, a predetermined region in the first or third quadrant in a two-dimensional display in the vertical (v) and time (t) directions would be used.
However, this system cannot be utilized in multiplexing an added signal for increasing the aspect ratio, because the added signal should be transferred for a motion picture. In the case of a motion picture, as the spectrum spreads in the time direction. If an added signal is multiplexed, therefore, it will be superimposed on the intrinsic NTSC signal, the added signal cannot be separated at the receiver side.
It is thus desirable to use a system which can transfer an added signal even for a motion picture. This system is disclosed in, for example, "Encoding for Compatibility in the ACTV System," by M. A. Isnardi et. al, IEEE Trans. on Broadcasting Vol. BC-33 No. 4 1987, pages 116-123.
This ACTV system subjects a main signal and an added signal to bandwidth compression to ensure multiplexing of the added signal for not only a still picture but also a motion picture.
Since the ACTV system significantly restricts the bandwidth in the vertical (v) and time (t) directions of the main signal and added signal, however, a reproduced image is likely to show unnatural motion or degradation of the vertical resolution. Specifically, according to the ACTV system the vertical upper limit frequency of a component of the main signal above 1.5 MHz is restricted to 525/4 (cph), a half of that involved in the NTSC system, in the case of a still picture (time frequency being 0 Hz). This also applies to a motion picture, and the vertical upper limit frequency in this case will be 0 (cph) with the time frequency being 15 Hz. According to the ACTV system, therefore, a reproduced image would be blurred due to a decrease in the diagonal resolution for a still picture and would significantly lose smooth movement for a motion picture.